54 THE FIRST DAY. [CHAP. 



notochord the body of the embryo appears somewhat opaque, 

 owing to the thickness of the medullary folds ; as these folds 

 slope away outwards on either side, so the opacity gradually 

 fades away in the pellucid area. There is present at the sides 

 no sharp line of demarcation between the body of the embryo 

 and the rest of the area ; nor will there be any till the lateral 

 folds make their appearance ; and transverse vertical sections 

 shew (Fig. 12) that there is no break in the mesoblast, from 

 the notochord to the margin of the pellucid area, but only a 

 gradual thinning. 



10. During the latter period of the day, however, the 

 plates of mesoblast on either side of the notochord begin to 

 be split horizontally into two layers, the one of which attach- 

 ing itself to the epiblast, forms with it the somatopleure (Fig. 

 13, compare also Fig. 20, So.), while the other, attaching itself 

 to the hypoblast, forms with it the splanchnopleure (Fig. 13, 

 Be, Fig. 20, sp). By the separation of these two layers from 

 each other, a cavity (Fig. 13, pp, and Fig. 20, pj)), containing 

 fluid only, and more conspicuous in certain parts of the 

 embryo than in others, is developed. This cavity is the be- 

 ginning of that great serous cavity of the body which after- 

 wards becomes divided into separate cavities. We shall speak 

 of it as the pleuro-peritoneal cavity. 



11. This cleavage into somatopleure and splanchnopleure 

 does not extend quite up to the walls of the medullary canal. 

 Hence there is left along either side of the canal, between it 

 and the line along which the cleavage begins, a tract or plate 

 of unci eft mesoblast, which receives the name of vertebral 

 plate, the more external mesoblast being called the lateral 

 plate. 



At first each vertebral plate is not only unbroken along 

 its length but also continuous at its outer edge with the 

 upper and lower layers of the lateral plate of the same side. 

 Very soon, however, clear transverse lines are seen, in surface 

 views, stretching inwards across each vertebral plate from the 

 lateral plate towards the notochord ; and not long after a 

 transparent longitudinal line makes its appearance on either 

 side of the notochord along the line of junction of the lateral 

 with the vertebral plate. 



These transparent lines are caused by the appearance of 

 vertical clefts, giving rise to narrow spaces containing nothing 



